Prince Harry and Meghan have joined the World Health Organization (WHO) and Save the Children in appealing to G20 leaders meeting this weekend to honour promises to send Covid-19 vaccines to low-income countries where just 3% of people have had a jab.

It is one of the most directly political initiatives at a high-profile political summit by the former royal couple since they left the British royal household.

In a two-page letter, the Duke and Duchess of Sussex, the WHO director-general, Tedros Adhanom Ghebreyesus, the Save the Children International CEO, Inger Ashing, and representatives of other health charities called for the world’s wealthiest countries to accelerate long-promised donations and break the hold pharmaceutical companies have over access to vaccines. The issue was being discussed at a G20 health ministers meeting in Rome where it was agreed to try to vaccinate 70% of all populations by mid-2022.

But the letter urges greater ambition, pointing out that “among countries represented at the G20, there are a handful with millions of surplus vaccines that are destined to be wasted once they expire,” the letter said.

“Every discarded dose of a Covid-19 vaccine, when there are the mechanisms to donate them, should outrage us all. Each dose represents a real person – a mother, father, daughter, or son – who could have been protected.”

They said 7bn doses of Covid-19 vaccines have been administered globally with some people now receiving three vaccines, but Covax – the initiative designed to help fairly achieve global access to vaccines – had only received 11.5% of the promised 1.3bn doses.

Global targets have been set to vaccinate 40% of the population of every country by the end of 2021 and 70% by mid-2022. It is expected that the G20 leaders’ summit in Rome this weekend will see that target brought forward, but previously some European powers have not met their vaccine pledges.

“By delivering already-pledged doses, helping countries manufacture their own vaccines, and prioritising vaccines for nations in need, the G20 can help ensure the world delivers on these promises,” said the letter that was also signed by the Vaccine Alliance, UNAIDS, Clinton Foundation and Global Citizen.

“We can’t simply hope for the pandemic to end on its own. As the virus progresses through unvaccinated populations, we risk new and more deadly strains sweeping the planet.”

The letter also calls for the G20 to immediately close a 550m-dose gap to accelerate towards WHO’s 40% coverage target by the end of 2021, by speeding up existing commitments of donations to Covax, pledging new ones, executing swaps with Covax, and eliminating export restrictions on vaccines.